Image Processing Reference
In-Depth Information
5.2 Design Challenges
In the future, many low-end sensor nodes will be deployed to minimize the cost of the sensor
networks. These nodes may work collaboratively together to provide time synchronization for the
whole sensor network. he precision of the synchronized clocks depends on the needs of the applica-
tions. For example, a sensor network requiring TDMA service may require microseconds difference
among the neighbor nodes while a data gathering application for sensor networks requires only
milliseconds of precision.
As sensor networks are application driven, the design challenges of a time synchronization
protocol are also dictated by the application. hese challenges are to provide an overall guideline and
requirement when considering the features of a time synchronization protocol for sensor networks;
they are robust, energy aware, server-less, lightweight, and tunable service.
•
Robust
: Sensor nodes may fail, and the failures should not have significant effect on the
time synchronization error. If sensor nodes depend on a specific master to synchronize
their clocks, a failure or anomaly of the master's clock may create a cascade effect that
nodes in the network may become unsynchronized. So, a time synchronization protocol
has to handle the unexpected or periodic failures of the sensor nodes. If failures do occur,
the errors caused by these failures should not be propagated throughout the network.
•
Energy aware
: Since each node is battery-limited, the use of resources should be evenly
spreadandcontrolled.Atimesynchronizationprotocolshouldusetheminimumnumber
of messages to synchronize the nodes in the earliest time. In addition, the load for time
synchronization should be shared, so some nodes in the network do not fail earlier than
others.Ifsomepartsofthenetworkfailearlierthanothers,thepartitionednetworksmay
drift apart from each other and become unsynchronized.
•
Server-less
: A precise time server may not be available. In addition, the time servers may
fail when placed in the sensor ield. As a result, sensor nodes should be able to synchronize
to a common time without the precise time servers. When the precise time servers are
available, the quality of the synchronized clocks as well as the time to synchronize the
clocks of the network should be much better. his server-less feature also helps to address
the robustness challenge as stated earlier.
•
Lightweight
: he complexity of the time synchronization protocol has to be low in order
to be programmed into the sensor nodes. Besides being energy-limited, the sensor nodes
arememorylimitedaswell.hesynchronizationprotocolmaybeprogrammedintoa
FPGAordesignedintoanASIC.Byhavingthetimesynchronizationprotocoltightly
integrated with the hardware, the delay and variation of the processing may be smaller.
With the increase of precision, the cost of a sensor node is higher.
•
Tunable service
: Some services such as medium access may require time synchronization
to be always ON while others only need it when there is an event. Since time synchro-
nization can consume a lot of energy, a tunable time synchronization service is applicable
for some applications. Nevertheless, there are needs for both type of synchronization
protocols.
The above challenges provide a guideline for developing various types of time synchronization
protocols that are applicable to the sensor networks. A time synchronization protocol may have a
mixture of these design features. In addition, some applications in the sensor networks may not
require the time synchronization protocol to meet all these requirements. For example, a data gath-
ering application may require the tunable service and lightweight features more than the server-less
capability. The tunable service and lightweight features allow the application to gather precise data
when the users require it. In addition, the nodes that are not part of this data gathering process may
